Exposure determining device

ABSTRACT

An exposure determining device comprises a bridge circuit which includes at least a photoelectric element for receiving light from an object to be photographed and a variable resistor for controlling the balance of the bridge circuit and associated with exposure factors. The device further comprises a bridge balance detecting circuit producing one of first and second signals upon unbalance of the bridge circuit, and an electric motor rotatable in forward and reverse directions by first and second driving circuits operable in response to the first and second signals, respectively. The motor is associated with the resistor in such a manner that the resistance value of the resistor is decreased during the rotation of the motor in one direction and is increased during the rotation of the motor in the other direction. The opposite terminals of the motor may be shortcircuited by a short-circuiting circuit. A relay circuit is provided for closing the first and second driving circuits in response to the first and second signals, respectively, and for closing the short-circuiting circuit upon balance of the bridge circuit.

United States Patent 1191 Maida 1451 May 7,1974

[ EXPOSURE DETERMINING DEVICE [75] Inventor: Osamu Maida, Tokyo, Japan [73 Assignee: Nippon Kagaku K.K., Tokyo, Japan [22] Filed: Nov. 2, 1972 [21] Appl. No.: 303,233

[30] Foreign Application Priority Data 3,110,851 11/1963 Plogstedt et a1. 318/294 X 3,444,446 5/1969 Burg et al. 318/612 X 3,581,181 5/1971 Wallacem, 318/640 X Primary ExaminerRobert K. Schaefer Assistant ExaminerW. E. Duncanson, Jr.

Attorney, Agent, or FirmFitzpatrick, Cella, Harper & Scinto Japan 46-96140 [5 7] ABSTRACT An exposure determining device comprises a bridge circuit which includes at least a photoelectric element for receiving light from an object to be photographed and a variable resistor for controlling the balance of the bridge circuit and associated with exposure factors. The device further comprises a bridge balance detecting circuit producing one of first and second signals upon unbalance of the bridge circuit, and an electric motor rotatable in forward and reverse directions by first and second driving circuits operable in response to the first and second signals, respectively.

The motor is associated with the resistor in such a manner that the resistance value of the resistor is decreased during the rotation of the motor in one direc tion and is increased during the rotation of the motor in the other direction. The opposite terminals of the motor may be short-circuited by a short-circuiting circuit. A relay circuit is provided for closing the first and second driving circuits in response to the first and second signals, respectively, and for closing the shortcircuiting circuit upon balance of the bridge circuit.

5 Claims, 2 Drawing Figures R9 RIO R EXPOSURE DETERMINING DEVICE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the stabilization of an automatic photographic exposure determining device having a feedback circuit commonly known as a servo circuit.

2. Description of The Prior Art I In a known automatic exposure determining device of the described type, the stabilization of the servo system has been accomplished by providing an internal feedback referred to as speed feedback thereby to reduce the constant speed of a servo motor. This may qualitatively be explained as follows: among the elements constituting the servo system, movable members such as a drive motor, reduction gearing, the member to be controlled, etc., have their own masses which, when moving, generate a moment ofinertia resulting in various phenomena such as hunting, overshoot, etc., which, in turn, aggravate the damping of the servo system. Therefore, speed feedback has been employed to reduce the constant speed of the motor and thereby reduce the effect of the moment of inertia, thus improving the damping. Generally, in the method employing speed feedback, a tachometer generator has been provided on the output shaft of the drive motor to detect the rotational speed of the output shaft and apply a feedback to the drive motor. This method, however, suffers from disadvantages in that the output of the drive motor is consumed into the rotation of the tachometer generator, that the use of such generator makes it difficult to providea compact construction, and that the constant speed reduced by the application of speed feedback leads to a reduced response speed.

SUMMARY OF THE INVENTION In view of the fact that short-circuiting of the armature winding on a compact DC motor causes an electromagnetic brake to be applied to the motor, and this acts effectively as a damper, the present invention intends to eliminate the above-noted disadvantages and stabilize the servo system by arranging the motor so that it acts as a drive motor during its energized condition and acts as a damper during its deenergized condition.

To achieve this object, the present invention provides an exposure determining device which comprises a bridge circuit including at least a photoelectric element adapted to receive the light from an object to be photographed and a variable resistance element for controlling the balance of said bridge circuit and associated with exposure factors. A circuit for detecting the balance of the bridge circuit is provided and produces one of first and second signals upon unbalance of the bridge circuit. An electric motor, rotatable in forward and reverse directions, is operatively associated with the resistance element in such a manner that the resistance value of the resistance element is decreased when the electric motor rotates in one direction and is increased when the electric motor rotates in the other direction. The device further comprises a circuit for shortcircuiting both terminals of the electric motor, a first driving circuit for rotating the electric motor in forward direction, the first driving circuit becoming operative upon receipt of the first signal, a second driving circuit for rotating the electric motor in reverse direction, the second driving circuit becoming operative upon receipt of the second signal, and a relay circuit for closing the first driving circuit into operative condition upon receipt of the first signal, closing the second signal and closing the short-circuiting circuit upon balance of the bridge circuit. Thus, the electric motor varies the resistance value of the resistance element by the rotation thereof upon unbalance of the bridge circuit and the motor is stopped by the short-circuiting circuit upon balance of the bridge circuit.

The relay circuit may include a relay for selectively closing the short-circuiting circuit and the two driving circuits. The relay closes the short-circuiting circuit upon balance of the bridge circuit, the first driving circuit upon receipt of the first signal and the second driving circuit upon receipt of the second signal, respectively.

The detecting circuit may be a complementary circuit composed of pnp and npn transistors, one of which selectively becomes conductive upon unbalance of the bridge circuit. Thus, the pnp transistor produces the first signal in the conductive condition thereof and the npn transistor produces the second signal in the conductive condition thereof.

There has thus been outlined rather broadly the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto. Those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures for carrying out the several purposes of the invention. It is important, therefore, that the claims be regarded as including such equivalent construction as do not depart from the'spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS A specific embodiment of the invention has been chosen for purposes of illustration and description, and is shown in the accompanying drawings, forming a part of the specification, wherein:

FIG. 1 is a circuit diagram showing a first embodiment of the present invention; and

FIG. 2 is a circuit diagram showing a second embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, there is shown a circuit according to a first embodiment of the present invention. It includes a photoelectric element Bph such as Ca'S cell, and a function resistor Rv operatively associated with a DC drive motor M in such a manner that the resistance value of the function resistor Rv is decreased when a current flows through the motor M in the direction of arrow 1'. The function resistor Rv is also operatively associated with exposure factors such as F- number and shutter time in such a manner that the combination of the exposure factors is varied in the direction toward under-exposure when the motor is driven in the direction for decreasing the resistance value of the function resistorRv', and that the combination of the exposure factors provides a proper exposure when the valve of the resistance Rv is equal to the value of the photoelectric element Rph.

' Resistors R3 and R4, equal in resistance value,'cooperate with the photoelectric element Rph and function resistor Rv to constitute a bridge circuit. An npn transistor T1 and a pnp transistor T2 have their respective bases connected with a balance output terminal a of the bridge circuit constituted by the photoelectric element Rph and function resistor Rv, and have their respective emitters connected with the other balance output terminal of the bridge circuit constituted by resistors R3 and R4. Thus, the transistors T1 and T2 constitute a complementary circuit which is a circuit for detecting the balance of the bridge circuit. The collector of the transistor T1 is connected with the base of a pnp transistor T3 through the resistor R1, and the collector of the transistor T2 is connected withthe base of an npn transistor T4 through the resistor R2. The collector of the transistor T3 is, in turn, connected with the collector of the transistor T4 through serially connected resistors R5 and R6. The emitters of the transistors T3 and T4 are connected with the positive and negative terminals of an electric power source Eb, respectively. An npn transistor T5 and a pnp transistor T6, connected together in complementary relationship, have their respective bases connected with the midpoint between the resistors R5 and R6, and have their respective emitters connected with one of the terminals of DC drive motor M (hereinafter referred to as motor M"). The collectors of the transistors T5 and T6 are connected with the positive and negative terminals of the electric power source Eb, respectively. Thus, the transistors T3, T4, T5 and T6 together constitute a drive circuit for the motor M. The other terminal of the motor M is connected with a common relay contact Sa which is operated by a relay coil L. A break contact Sb, adapted to be closed when there is no current flowing to the relay coil L, is connected with the emitters of the transistors T5 and T6, and a make contact Sc, adapted to be closed when there is a current flowing to the relay coil L, is connected with the midpoint ofthe electric power source Eb. The opposite terminals of the motor M are short-circuited when there is no current flowing to the relay coil L. A pnp transistor T9 has its emitter connected with the positive terminal of the electric power source Eb, its base connected with the collector of the transistor T1 through a resistor R7, and its collector connected through a resistor R11 with an npn transistor T8 for energizing the relay coil L. An npn transistor T has its emitter connected with the terminal of the electric power source Eb, its base connected with the collector of the transistor T2 through a resistor R8, and its collector connected with the base of a pnp transistor T7 through a resistor R10. The emitter of the transistor T7 is connected with the positive terminal of the electric power source Eb and the collector thereof is connected with the base of the transistor T8 through a resistor R9'. The emitter of the transistor T8 is connected with the negative terminal of the electric power source Eb and the collector thereof is connected with one end of the relay coil L, whose other end is, in turn, connected with the positive terminal of the electric power source Eb. The transistors T7, T8, T9, T10 and the relay coil L together constitute a relay energizing circuit for selectively closing the common relay contact Sa with respect to the break contact Sb or to the make contact Sc.

Operation of the described circuit of the present invention will now be explained.

When the photoelectric element Rph receives light, it presents a-resistance as determined by its own characteristic. If the resistance value is smaller than that of the function resistor Rv, i.e., if the exposure determined by the combination of exposure factors such as F-number, shutter speed, film sensitivity, etc. is in a condition of over-exposure, then the potential at the balance output terminal point a will be higher than the potential at the balance output terminal point b. If the difference between the two potentials is greater than the threshold voltage between the base and emitter of the transistor T1, this transistor T1 will become conductive and accordingly, the transistor T3 will also become conductive. Since the transistor T2 is in nonconductive state, the transistor T4 will also be nonconductive. Thus, the transistor T5 is ready to be driven. At the same time, the transistor T9 willbecome conductive to detect the readiness of the motor M to be drivenand accordingly, the transistor T8 will conduct to energize the relay coil L. Energization of the relay coil L will cause the common relay contact Sa to be closed with respect to make contact Sc.-A current will flow through the motor M in the direction of arrow 1', to rotate the motor M, whereupon the function resistor Rv is driven in a direction to decrease its resistance value while the exposure factors are driven in a direction toward under-exposure. When the potential difference between the points a and b becomes smaller than the threshold voltage between the emitter and base of the transistor T1, the transistors T 1 and T2 will become non-conductive. As a result, all transistors will become non-conductive to nullify the driving current to the motor M and also nullify the current to the relay coil L, so that the common relay contact Sa will be closed with respect to the break contact Sb, thus shortcircuiting the opposite terminals of the motor M to apply an electromagnetic brake to stop the motor quickly.

If the resistance value of the photoelectric element Rph is higher than that of the function resistor Rv, i.e., if the exposure determined by the combination of expo sure factors such as F-number, shutter speed, film sensitivity, etc. is in a condition of under-exposure, the potential at the point a will become lower than the potential at the point b. When the potential difference between the two points isgreater than the threshold voltage between the base and emitter ofthe transistor T2, this transistor T2 will become conductive and accordingly, the transistor T4 will also become conductive. Since the transistori-Tl is in non-conductive state, the transistor T3 will; also be nonconductive. Thus, the transistor T6 is ready to be driven. At the same time, the transistor T10 will become conductive to detect the readiness of the motor M to be driven and accordingly, the transistors T7 and T8 will conduct to energize the relay coil L. Energization of the relay coil L will cause the common relay contact Sa to be closed with respect to the make contact Sc, thus permitting a current to I flow through the motor M in the direction of arrow between the points a and b becomes smaller than the threshold voltage between the emitter and base of the transistor T2, the transistors T1 and T2 will become nonconductive and thus, all transistors will become nonconductive. Thereupon, the opposite terminals'of the motor M will be short-circuited to apply an electromagnetic brake to stop the motor quickly. In this way, the drive motor M acts as a damper when it is stopped, and this provides a very good damping of the servo system.

FIG. 2 shows another embodiment of the present invention in which the driving circuit for motor M is provided by a single electric power source. I

In the circuit of this embodiment, if the potential at the balance output terminal point a is higher than the potential at the point b and the potential difference becomes greater than the threshold voltage between the base and emitter of transistor T1, then npn transistor T1, for detecting the balance of the bridge, will become conductive. This renders conductive przp transistor T3, npn transistor T6, npn transistor T8 and pnp transistor T9 which constitute a motor driving circuit, as well as npn transistor T12 and pnp-transistor T1 1 which constitute a relay energizing circuit. On the other hand, pnp transistor T2, for detecting the balance of the bridge, is in non-conductive state, thus rendering. non-conductive npn transistor T4, pnp transistor T5, pnp transistor T7 and npn transistor T10 which constitute the motor driving circuit. As a result, relay coil L is energized to cause a common relay contact Sa to be closed with respect to a make contact Sc, permitting a current to flow through motor M in the direction of arrow As the motor M is rotated, the function resistor Rv is driven in a direction todecrease its resistance value and the exposure factors are driven in a direction toward under-exposure. If the potential at the point a becomes lower than that at the point b and the potential difference is greater than the'threshold voltage between the base and emitter of the transistor T2, this transistor T2 will become conductive. Thus, transistors T4, T5, T7, T10 and T11 will all become conductive. Since the transistor T1 is in non-conductive state, transistors T3, T6, T8, T9" and T12 are also in non-conductive state. Therefore, the relay coil L will be energized and the common relay contact Sa will be closed with respect to the make contact Sc to permit a current to flow through the motor M in the direction of arrow i thus rotating the motor M. At the same time, the function resistor Rv will be driven in a direction to increase its resistance value while the exposure factors will be driven in a direction toward over-exposure. When the potentials at the points a and b become equal, all transistors will be rendered non-conductive to deenergize the relay coil L, so that the common relay contact Sa will be closed with respect to the break contact Sb, thus shortcircuiting the opposite terminals of the motor M to apply an electromagnetic brake to stop the motor quickly.

With the circuit so-constructed and operable as described above, the aggravated damping effect (i.e., in stability of the servo system) resulting from the moment of inertia of the movable members forming the servo system may be prevented by applying an electromagnetic brake through the short-circuiting of the motor armature winding to improve the damping effect and achieve the stabilization of the servo system. More- 6 over, any special damper means, speed feedback and the like are not required and this leads to the advantages that the constant speed need not be reduced (that is, any reduction in response speed may be obviated) and that the entire device may be made compact.

in the first and second embodiments of the present invention, the relay energizing circuit has been described as being controlled by using the collector current of the balance detecting transistors T1, T2 and the collector current of the transistors T3, T4 in the motor driving circuit, respectively, to detect the readiness of the motor driving circuit to drive the motor; whereas these are not the only means of control, but it will be apparent that other circuit arrangements may be available if only the readiness of the motor to be driven is detected to control the relay circuit.

Further, in the above-described embodiments, automatic exposure control is accomplished by driving the function resistor from the motor thereby to balance the bridge circuit, but will will also be apparent that the effect of the present invention may equally be achieved by providing a diaphragm, filter or like means in front of the photoelectric element and causing the motor to displace such means thereby to vary the resistance value of the photoelectric element so as to balance the bridge circuit.

I believe that the construction and operation of my novel exposure determining device will now be understood, and that its advantages will be fully appreciated by those persons skilled in the art.

l claim:

1. An exposure determining device comprising;

an exposure detecting circuit including a photoelectric element adopted to receive light from an object to be photographed, the exposure detecting element producing a first signal in a case of underexposure, a second signal in a case of overexposure and a third signal in a case of proper exposure,

relay switch means having a first state and a second state, means for switching said relay switch means to said first state in accordance with said first signal and for switching said relay switch means to said second state in accordance with said third signal,

means for setting'at least one exposure factor,

an electric motor rotatable in forward and reverse directions and connected to said exposure factor setting means to attain proper exposure,

a first motor driving circuit including said relay switch means and a first transistorized switch connected in series with said relay switch means, said first motor driving circuit being operative when said relay switch means is in said first state and when said first transistorized switch is on in accordance with said first signal,

a second motor driving circuit for the reverse rotation of said motor including said relay switch means and a second transistorized switch con-- nected in series with said relay switch means, said second motor driving circuit being operative when said relay switch is in said first state and when said second transistorized switch is on in accordance with said second signal, and

a circuit for short-circuiting both terminals of said motor when said relay switch means is in said second state.

7 8 2. An exposure determining device according to 5. An exposure determining device according to claim I, wherein said exposure detecting circuit inclaim 1, wherein said relay switch 'means is connected cludes 1 to said motor, I a bridge circuit having a variable resistor associated said first motor driving circuit includes a transistor with said exposure factor setting means and a pho- 5 which functions as a third switch means and is on toelectri'c element receiving light from the object upon receipt of said first signal, to be photographed, and said second motor driving circuit includes a transistor a complementary circuit including PNP and NPN which functions as a fourth switch means and is transistors for detecting the state of the bridge ciron upon receipt of said second signal, cuit. said first and second motor driving circuits havethe 3. An exposure determing device according to claim common power source (E' 1, wherein said relay switch switching means includes said first switch means (T is connected between a transistor connected in series with a coil of said relay the positive terminal of the power source and a terswitch means, said transistor being conductive-upon reminal of the motor, ceipt of said first or second signal. 5 said third switch means (T is connected between 4. An exposure determining device according to the negative terminal of the power source and the claim l,.wherein said first motor driving circuit inrelay switch means, cludes a first power source connected in series with said second switch (T is connected between 'the said relay switch means and with said first switch negative terminal of the power source and the relay means, and i switch means, and

said second motor driving circuit includes a second said fourth switch means (T' is connected between power source connected in series with said relay the positive terminal of the power source and the switch means and with said second switch means. relay switch means.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3, 809,98l Dated May 7 1974 Patent Nu Inventor) OSAMU MAIDA It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 58, change "Bph" to Rph Column 3, line 3, change "valve" to value Column 6, line 43, after "first (second occurrence),

insert or second Signed and sealed this 29th day of October 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents FORM P0405) (1069) v USCOMM-DC 6O376-P69 VI U.S. GOVERNMENT PRINTING OFFICE I969 0-366-33L UNITED STATES PATENT OFFICE I CERTIFICATE OF CORRECTION PIILI III No. I ,809,98l M May 7, 1974 lhventofls) I OSAMU MAIDA It certified that error appears in the above-identified petent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 58, change "Bph" to Rph Column 3, line 3,' change "valve" to value Column 6, line 43', after "first" (second occurrence) insert or second Signed a nd sealed this 29th day of October 1974.

EAL) Attest MCCOY M. GIBSON JR; I c. MARSHALL DANN Attesting Officer Commissioner of Patents FORM USCO'MM-DC 60376'P69 I I I I I v: u.s. sovnmasm rnm'rms OFFICE: 19 o-ass-su, 

1. An exposure determining device comprising; an exposure detecting circuit including a photoelectric element adopted to receive light from an object to be photographed, the exposure detecting element producing a first signal in a case of under-exposure, a second signal in a case of over-exposure and a third signal in a case of proper exposure, relay switch means having a first state and a second state, means for switching said relay switch means to said first state in accordance with said first signal and for switching said relay switch means to said second state in accordance with said third signal, means for setting at least one exposure factor, an electric motor rotatable in forward and reverse directions and connected to said exposure factor setting means to attain proper exposure, a first motor driving circuit including said relay switch means and a first transistorized switch connected in series with said relay switch means, said first motor driving circuit being operative when said relay switch means is in said first state and when said first transistorized switch is ''''on'''' in accordance with said first signal, a second motor driving circuit for the reverse rotation of said motor including said relay switch means and a second transistorized switch connected in series with said relay switch means, said second motor driving circuit being operative when said relay switch is in said first state and when said second transistorized switch is ''''on'''' in accordance with said second signal, and a circuit for short-circuiting both terminals of said motor when said relay switch means is in said second state.
 2. An exposure determining device according to claim 1, wherein said exposure detecting circuit includes a bridge circuit having a variable resistor associated with said exposure factor setting means and a photoelectric element receiving light from the object to be photographed, and a complementary circuit including PNP and NPN transistors for detecting the state of the bridge circuit.
 3. An exposure determing device according to claim 1, wherein said relay switch switching means includes a transistor connected in series with a coil of said relay switch means, said transistor being conductive upon receipt of said first or second signal.
 4. An exposure determining device according to claim 1, wherein said first motor driving circuit includes a first power source connected in series with said relay switch means and with said first switch means, and said second motor driving circuit includes a second power source connected in series with said relay switch means and with said second switch means.
 5. An exposure determining device according to claim 1, wherein said relay switch means is connected to said motor, said first motor driving circuit includes a transistor which functions as a third switch means and is ''''on'''' upon receipt of said first signal, said second motor driving circuit includes a transistor which functions as a fourth switch means and is ''''on'''' upon receipt of said second signal, said first and second motor driving circuits have the common power source (E''b), said first switch means (T''7) is connected between the positive terminal of the power source and a terminal of the motor, said third switch means (T''10) is connected between the negative terminal of the power source and the relay switch means, said second switch (T8) is connected between the negative terminal of the power source and the relay switch means, and said fourth switch means (T''9) is connected between the positive terminal of the power source and the relay switch means. 